Geological exploration of the sea bed is generally performed by means of ship towing acoustic emission systems such as the system specified above, which systems emit soundwaves in different directions towards the bed to be explored. The soundwaves are then reflected by the formations under the sea bed and they are detected by transducers disposed along cables, known as "streamers", and likewise towed by the prospecting ship. By analyzing the reflected waves relative to the waves emitted by the sources, information is obtained on the structure, the nature, and the composition of the sea bed being studied.
The sources used in acoustic emission systems for seismic surveying at sea often constitute subassemblies forming a regular array in one to three dimensions. To emit a wave in a given direction, emission from the sources is controlled in application of a phase law that governs the phase delays to be applied to the sources. A nominal phase law is defined for an ideal configuration of the sources in which the water surface is planar and completely calm. For example, under such conditions in order to emit a wave in the vertical direction perpendicular to the surface of the water, the nominal phase law is uniform in that all of the sources emit simultaneously. In contrast, a soundwave of given inclination can be obtained by offsetting emissions from consecutive sources relative to one another by a phase delay that is a function of the looked-for inclination, it being understood that the emission direction is the direction in which the waves emitted by all of the sources interfere constructively.
In practice, real prospecting conditions are significantly different from the above-mentioned ideal configuration for which the nominal phase law is defined. In particular, the surface of the sea often has short range instabilities such as waves whose space frequency is equal to or greater than the distance between two consecutive sources within a subassembly under consideration. Such surface instabilities give rise to the sources moving with pendulum motion and without correlation between one another, thereby having the effect of varying the depth of each source as a function of time. The positions of the sources relative to the surface are thus affected, with the consequence that the nominal phase law no longer corresponds to the real configuration of the sources. The nominal phase law must therefore be corrected in real time. To do this, each source may be fitted with a pressure gauge for example, thereby providing a measure at each instant of the exact depth of each source relative to the surface, thereby making it possible to establish the relative positions between the sources. The necessary corrections to the phase law so as to take into account instabilities of this type are then deduced therefrom.
A method using such depth measurement, i.e. a depth of water measured piezometrically and corresponding to the effective towing depth of each source, is described in GB-2 148 001.
However, that known correction method is not usable when the water surface although calm, i.e. not having large waves, nevertheless presents undulations of large space period due to the phenomenon of swell. Under such circumstances, the sources remain relatively steady relative to the surface and the pressure gauges of the sources do not record any variations in pressure and thus in depth. Consequently, the nominal phase law is not corrected even though the sources are not in the corresponding ideal configuration.
Thus, the technical problem to be solved by the present invention is that of providing an acoustic emission system for seismic surveying at sea and as defined in the introduction, which system should make it possible to correct the nominal phase law of emission from the sources with respect to low frequency movements of the water surface.